HashSet
Java collections can be divided into three kinds of large systems Set.List and Map
HashSet is a typical implementation of the Set interface, most of the time using the Set uses this implementation class when the collection.
Most of the time we say that the set is a collection of finger HashSet
HashSet press Hash algorithm stored in the elements of the set, it has a good storage and lookup performance.
HashSet has the following characteristics
1 can not guarantee the order of the elements
2. the non-repeatable
3.HashSet not thread safe
4. the element may be a null set
HashSet does not guarantee data storage location, there is hashcode decide which position Set collection of this value.
import java.util.HashSet;//在newHashSet时会自动生成
import java.util.Set;//import接口Set
public class Test02 {
public static void main(String[] args) {
Set set = new HashSet<>();
set.add(1);//使用add给集合添加元素
set.add("a");
System.out.println(set);//直接打印对象set就可以 [1,a]
set.remove(1);//移除元素
System.out.println(set);//[ture]
set.contains("a");//判断是否包含元素
System.out.println(set.contains("a"));//ture
set.clear();//清除元素
System.out.println(set);//[]
}
}
Using the method iterates over the collection:
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
public class Test02 {
public static void main(String[] args) {
Set set = new HashSet<>();
set.add("a");
set.add("b");
set.add("c");
set.add("d");
//使用迭代器遍历集合
Iterator it = set.iterator();
while(it.hasNext()) {
System.out.println(it.next());
}
//for each 迭代集合
for(Object obj : set) {//这个的意思是把set的每一个值取出来,赋值给obj,知道循环set的所有值
System.out.println(obj);
}
System.out.println(set.size());//获取集合的元素个数
}
}
hashcode () method
HashSet set equal to the judgment criteria two elements: two objects by equals () method for determining equal, and the two objects hashCode () method returns a value equal.
Returns true if two objects by equals () method these two hashCode value of the object should be equal
Generic
using Set <> set1 = new HashSet < >;
to specify the engagement element HashSet centrally stored, in addition to the type of fill within brackets, the other can no longer store
for example:
the Set <String> = setl new new HashSet <String>; // string type can only store strings
TreeSet
TreeSet SortedSet is a class that implements the interface, TreeSet ensure collection of elements in the sort state.
TreeSet supports two sorting methods: natural order and custom sorting By default, TreeSet uses natural ordering.
import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;
public class Test03 {
public static void main(String[] args) {
Set<Integer> set = new TreeSet<Integer>();
set.add(5);
set.add(2);
set.add(4);
set.add(3);
System.out.println(set);//[2,3,4,5] TreeSet自然排序
set.remove(5);//移除方法
set.contains(3);//判断方法
// set.clear();//清除方法
//使用迭代器遍历
Iterator<Integer> it = set.iterator();
while(it.hasNext()) {
System.out.println(it.next());
//for each迭代集合
for(Integer i : set) {
System.out.println(i);
}
}
}
Custom ordering
the Person object saved to TreeSet and sorted by age
class Person implements Comparator<Person>{//import java.util.Comparator;
int age;
String name;
public Person(){//空构造器
};
public Person(int age,String name) {
this.age = age;
this.name = name;
}
@Override
public int compare(Person o1, Person o2) {//年龄正序排列
if(o1.age > o2.age) {
return 1;
}else if(o1.age < o2.age) {
return -1;
}else{
return 0;
}
}
}
import java.util.Comparator;
import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;
public class Test03 {
public static void main(String[] args) {
Person p1 = new Person(27,"zhangsan");
Person p2 = new Person(22,"zhangsan1");
Person p3 = new Person(45,"zhangsan2");
Person p4 = new Person(34,"zhangsan3");
Set<Person> set = new TreeSet<Person>(new Person());
set.add(p1);
set.add(p2);
set.add(p3);
set.add(p4);
for(Person p : set) {
System.out.println(p.age + " " + p.name);
}
}
}
List与ArrayList
import java.util.ArrayList;
import java.util.List;
public class Test04 {
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("b");//第一个,索引下标0
list.add("d");//索引下标1
list.add("c");//索引下标2
list.add("a");//索引下标3
list.add("d");//索引下标4,允许使用重复元素
System.out.println(list);//[b, d, c, a, d]
System.out.println(list.get(2));//获取下标为2的元素,输出结果:c
list.add(1,"f");//在第1个索引下标位置插入元素 f
System.out.println(list);//[b, f, d, c, a, d]
List<String> l = new ArrayList<String>();
l.add("123");
l.add("456");
list.addAll(2, l);//在第二个index位置添加list l
System.out.println(list);//[b, f, 123, 456, d, c, a, d]
//获取指定元素在集合中第一次出现的索引下标
System.out.println(list.indexOf("d"));//索引下标第五位,输出结果:4
//获取指定元素在集合中最后一次出现的索引下标
System.out.println(list.lastIndexOf("d"));//索引下标第八位,输出结果:7
list.remove(2);//根据指定索引下标移除元素
System.out.println(list);//[b, f, 456, d, c, a, d]
list.set(1, "ff");//根据指定的索引下标index修改元素
System.out.println(list);//[b, ff, 456, d, c, a, d]
//创建新的String类型List合集 sublist,根据索引下标的位置截取一段元素形成一段新的集合
List<String> sublist = list.subList(2, 5);//截取的时候包含开始的索引不包含结束时的索引 2,5 是大于等于3,小于等于6的元素.
System.out.println(sublist);//[456, d, c]
System.out.println(list.size());//获取集合的长度,输出结果:7
}
}
Map
Map for storing data having a mapping relationship, and therefore the collection Map holds two sets of values, a set of values stored in the Map Key, another group for holding the Map Value of
the Map and the key value can be any reference data type, Map of Key must be unique , that is, any two Key equals method returns false comparison with a Map object
exists and the one-way-one relationship between a Key Value, i.e., by setting the total energy Key find unique, determined Value
Map interface HashMap
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
public class Test05 {
public static void main(String[] args) {
Map<String, Integer> map = new HashMap<String,Integer>();
map.put("b", 1);//使用.put添加key
map.put("c", 2);
map.put("e", 2);//value可以相同,key不可以相同
System.out.println(map);//{b=1, c=2, e=2}
System.out.println(map.get("b"));//根据Key取值,输出结果:1
map.remove("c");//根据Key"c",移除键值对
System.out.println(map);//{b=1, e=2}
System.out.println(map.size());//获取map的长度,输出结果:2
System.out.println(map.containsKey("a"));//判断当前的map集合是否包含指定的Key,输出结果:false
System.out.println(map.containsValue(2));//判断当前的map集合是否包含指定的value,输出结果为:true
map.clear();//清空集合
Set<String> keys = map.keySet();//获取map集合的key的集合
map.values();//获取map集合的value值
//遍历map集合
for(String key : keys) {
System.out.println("Key " + key + ", value" + map.get(key));
//输出结果:Key b, value1 Key e, value2
}
//通过map.entrySet();遍历map集合
Set<Entry<String, Integer>> entrys = map.entrySet();//导入包 java.util.Map.Entry
for(Entry<String, Integer> en : entrys) {
System.out.println("key: " + en.getKey() + ", value:" + en.getValue());
//key: b, value:1
//key: e, value:2
}
}
}
